To continue our discussion of the basic types of lens filter, the other two basic types are listed below. These two types are not used as frequently as the previous ones.
Another popular choice among landscape photographers are rectangular filters. These filters are mounted just like square filters, via a filter holder system. It is impractical for graduated neutral density filters to be circular (due to different sizes of high-contrast areas and composition), so rectangular filters are the primary choice for landscape photography. And unlike square filters, there is more room to move up and down. The most popular size is 4×6, though larger and smaller sizes are available.
These filters are used inside long telephoto lenses, due to the large size of the front lens assembly. Only clear and polarizing filters are available as drop-in filters.
Next time we will go into more details. Until then, Happy Shuttering!